The Nd—Fe—B magnetic material is named as “Magnet King” because of its high magnetic energy and coercive force. It is used widely in fields such as electronics, computers, vehicles, machinery, energy and medical equipment. According to 1997 worldwide production statistics, 10,450 tons of Nd—Fe—B series permanent-magnet material was produced, including 8,550 tons of sintered Nd—Fe—B series magnet and 1,900 tons of bonded Nd—Fe—B series magnet. The sintered Nd—Fe—B series magnet has played an important role in the fields mentioned above. The book Ultra-strong Permanent Magnet by Zhou Shouzeng (Metallurgy Industry Press, 2004) introduced the following production process flow of sintering Nd—Fe—B series permanent-magnet material: Raw Material Preparation—Smelting—Casting—Crushing and Powdering—Magnetic Field Orientation, Molding—Sintering+Tempering+Machining, and Surfacing—Testing. The magnetic performance of sintering Nd—Fe—B series permanent-magnet alloy is very sensitive to the sintering and tempering factors. The magnetic performance of alloys with the same components can vary greatly from several times to tens of times or even to hundreds of times depending on the different sinter and temper processes. Therefore, it is very important to understand the effects of the heat treatment temper process on magnetic performance.
The temper process of sintered Nd—Fe—B permanent magnets can include a primary or a secondary temper treatment of the sintered and cooled Nd—Fe—B permanent-magnet blank. In the primary treatment, the sintered and cooled Nd—Fe—B permanent-magnet blank is heated to the temper temperature in the heating chamber of the vacuum furnace and insulated (held therein for a desired time). Then argon, nitrogen, or another inert gas is charged to the cooling chamber of the furnace for air-quench cooling the blank. The secondary treatment follows the same process as the primary temper treatment, but after the air-quench cooling, the material is heated to the second temper temperature and held at such temperature for a desired time. Then, the argon, nitrogen, or another inert gas is charged for air-quench cooling the material again.
Temper treatment can significantly improve the magnetic performance of the Nd—Fe—B permanent magnet, especially its coercive force. Better magnetic performance can be obtained if the alloy is cooled down immediately after the temper treatment. However, current technology is limited because the argon, nitrogen, or another inert gas used in the existing temper process is under normal (or atmospheric) pressure. The pressure of a fixed-volume ideal gas at a constant-temperature is directly proportional to molar numbers of the gas (i.e., Dalton's Law). Thus, in the current technology, the molar numbers of the inert gas, as a cooling exchange carrier under normal pressure, are relatively less, the cooling speed is relatively low, the intrinsic coercive force within the magnet cannot be effectively increased, and the excellent consistency of the intrinsic coercive force cannot be reached.